CVE-2026-98101ipv6: mcast: use copy-on-write RCU updates in ip6_mc_source()

Official description

In the Linux kernel, the following vulnerability has been resolved: ipv6: mcast: use copy-on-write RCU updates in ip6_mc_source() pmc->sflist is read locklessly under rcu_read_lock() by inet6_mc_check() during packet reception in the UDP and RAW multicast receive paths. ip6_mc_source() mutated psl->sl_addr and psl->sl_count in-place when adding or removing a source filter. Additionally, when expanding the filter buffer, newpsl was published via rcu_assign_pointer() before writing the new source into the array. Because 16-byte struct in6_addr writes are not atomic and array shifting is not synchronized with RCU readers, concurrent readers in inet6_mc_check() could read torn IPv6 addresses or observe duplicated/missed source entries. Fix this by switching ip6_mc_source() to copy-on-write RCU updates: allocate and fully populate newpsl before publishing it via rcu_assign_pointer(), and reclaim the old filter via kfree_rcu(), matching ip6_mc_msfilter(). Also remove the now unused IP6_SFBLOCK macro.

Affected packages & fixed versions

Affected products (as reported)

References

• https://git.kernel.org/stable/c/ac51321d3b2391860a935820ebcf4d8b9bb16a8a
• https://git.kernel.org/stable/c/dba00514bf668c26cb4900e9f0da84bf96bf065a
• https://git.kernel.org/stable/c/2c2091e2ee93049fc513ad1e6c99d8b6c809f467
• https://git.kernel.org/stable/c/20db91a052332ef5552bd2f651ffb9db911cf67b
• https://git.kernel.org/stable/c/c073d1b070f171d206b19c98d71739a97f15b3f1